A motorcycle is a vehicle which has excellent handling features but whose loading capability is somewhat restricted. A passenger may sit beneath the motorcycle driver, on the same bench as him, but this is an uncomfortable arrangement for travel of long distances on the road. Furthermore, such a seating arrangement is not even desirable in several instances, e.g. when the passenger is a female dressed with a skirt or dress.
This may be one reason why the side-car has been developed. Moreover, the side car will increase the total number of persons transported by the motorcycle, since one passenger can still sit behind the pilot on the same bench while a third person sits in the side-car. Side-cars can be disconnected from their motorcycle, and thus, the motorcycle may still be used without the side-car whereby the motorcycle can be used in two different modes.
A side car conventionally consists of a frame, carried by a single ground engaging wheel carried in a vertical position, and a lateral coupling means wherein the side car of the wheel will substantially be in the travelling direction of the motorcycle-side car vehicle. The side car box is usually positioned laterally of the motorcycle, intermediate the rear wheel of the motorcycle and the single wheel of the side car, wherein the side car box ground clearance can be reduced. Hence, the center of gravity of the vehicle is lowered, thus increasing its stability/handling properties. Additionally, this permits to desirably increase the overall dimensions of side car wheel, e.g. by a diameter of a value equal to that of the rear wheel of the motorcycle: a wheel having a large diameter improves the road going comfort of the vehicle, as is well known.
In the early types of side cars, their frames were fixedly secured to the associated motorcycle frames, thereby requiring modification in the motorcycle handling negotiates a bend, since the motorcycle could not then roll inside the curve. Moreover, vehicle stability was compromised in turns, since the radius of curvature of the motorcycle front wheel and of the side car wheel were substantially different from one another because they were laterally spaced apart, particularly when the motorcycle was changing speeds.
In view thereof, jointed couplings of the deformable parallelogram type were developed, to bias the side car wheel to laterally roll concurrently with the motorcycle rear wheel, in the same direction, when the vehicle in motion negotiation a bend. Such a vehicle is described e.g. in U.S. Pat. No. 4,385,770 issued in 1983. A problem with such an arrangement is that the side car box, and thus its passenger, cannot roll with the wheel inside the curve, contrarily to the motorcycle frame and to the pilot who will be able to roll with the wheels of the motorcycle. Thus, a non-negligeable part of the moving vehicle load negotiation a bend, will sustain a centrifugal load that increases as the lateral inclination (roll) of the side car wheels and motorcycle wheels is pronounced, leading to a drastic reduction in the handling and stability of the vehicle, particularly during lateral wind drafts. Indeed, the motorcycle pilot may compensate the centrifugal force applied onto the motorcycle, during bend negociation, by a centripetal force generated by the roll inclination interiorly of the curve of the vehicle path, i.e. the displacement of its center of gravity to the interior of the tangential point of contact of the motorcycle wheels with the ground. On the other hand, since the side car box is locked into an intermediate position sunk between the side car wheel and the motorcycle rear wheel, it is impossible for the side-car passenger who would like to contribute in the stabilization of the vehicle (by laterally shifting his weight on the interior side of the curve of the vehicle in motion) to effectively compensate for the centrifugal force applied onto the side car box, because he cannot, in his seating position within the side car box, shift his weight, and thus the center of gravity of the side car, on the other side of the side car wheel or of the rear wheel of the motorcycle, depending on the respective side of the motorcycle roll inclination.